Method of forming alternating regions of Si and SiGe or SiGeC on a buried oxide layer on a substrate

ABSTRACT

Semiconductor structures and methods of forming semiconductor structures, and more particularly to structures and methods of forming SiGe and/or SiGeC buried layers for SOI/SiGe devices. An integrated structure includes discontinuous, buried layers having alternating Si and SiGe or SiGeC regions. The structure further includes isolation structures at an interface between the Si and SiGe or SiGeC regions to reduce defects between the alternating regions. Devices are associated with the Si and SiGe or SiGeC regions.

FIELD OF THE INVENTION

The invention relates to semiconductor structures and methods of formingsemiconductor structures, and more particularly to structures andmethods of forming SiGe and/or SiGeC buried layers for CMOS/BiCMOS andpassive devices over Si or SOI substrates.

BACKGROUND DESCRIPTION

Generally, metal-oxide semiconductor transistors include a substratemade of a semiconductor material, such as silicon. The transistorstypically include a source region, a channel/well region and a drainregion within the substrate. The channel/well region is located betweenthe source and the drain regions. A gate stack, which usually includes aconductive material, a doped poly Si, a gate oxide layer and sidewallspacers, is generally provided above the channel/well region. Moreparticularly, the gate oxide layer is typically provided on thesubstrate over the channel region, while the gate conductor is usuallyprovided above the gate oxide layer.

The amount of current flowing through a channel is generally directlyproportional to the mobility of the carriers in the channel. Thus, byincreasing the mobility of the carriers in the channel, the operationspeed of the transistor can be increased. Also, mechanical stresseswithin a semiconductor device can modulate device performance by, forexample, increasing the mobility of the carriers in the semiconductordevice. Thus, to improve the characteristics of a semiconductor device,tensile and/or compressive stresses are created in the channel of then-type devices (e.g., NFETs) and/or p-type devices (e.g., PFETs).However, the same stress component, for example tensile stress orcompressive stress, improves the device characteristics of one type ofdevice (i.e., n-type device or p-type device) while discriminativelyaffecting the characteristics of the other type device. Also, somedevices do not require tensile and/or compressive stresses in thesubstrate.

Also with the progress of CMOS scaling, the distance from source todrain is greatly shortened. Therefore the doping of source/drain and theLDD (lightly doped drain) have to be limited to prevent device punchthrough. This limitation requires source/drain and extension dosereduction, which will impact the device performance. Because of this,there is a need to keep the same doping level in the source/drain andextension but confine the dopant diffusion.

While known methods provide structures that have tensile stressesapplied to the NFET device and compressive stresses applied along thelongitudinal direction of the PFET device, they may require additionalmaterials and/or more complex processing, and thus, resulting in highercost. Thus, it is desired to provide more cost-effective and simplifiedmethods for creating large tensile and compressive stresses in thechannels NFETs and PFETs, respectively.

SUMMARY OF THE INVENTION

In a first aspect of the invention, an integrated structure comprisesdiscontinuous, buried layers having alternating Si and SiGe or SiGeCregions. The structure further includes isolation structures at aninterface between the Si and SiGe or SiGeC regions to reduce defectsbetween the alternating regions. Devices are associated with the Si andSiGe or SiGeC regions.

In another aspect of the invention, an integrated structure comprises asubstrate and alternating Si and SiGe or SiGeC regions formed on thesubstrate. Isolation structures are at an interface between the Si andSiGe or SiGeC regions. A buried oxide (BOX) layer is formed above thesubstrate. A first type device is associated with the Si regions and asecond type device is associated with the SiGe or SiGeC regions.

In yet another aspect of the invention, a method of forming anintegrated structure comprises forming SiGe or SiGeC regions alternatingwith Si regions on a substrate from continuous layer of SiGe or SiGeCmaterial or Si material formed over the substrate. The method furtherincludes forming isolation structures at interfaces between the SiGe orSiGeC regions and the alternating Si regions. A buried oxide layer isformed over the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 show structures according to a first embodiment of theinvention;

FIGS. 7-13 show structures according to a second embodiment of theinvention;

FIGS. 14-22 show structures according a third embodiment of theinvention;

FIG. 23 shows a schematic representation of a band alignment inaccordance with an implementation of the invention;

FIG. 24 shows a graph of out diffusion of Boron in a structureimplementing aspects of the invention;

FIG. 25 shows a final structure according to an aspect of the invention;and

FIG. 26 shows a final structure according to an aspect of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

This invention is directed to semiconductor structures and methods ofmanufacturing the semiconductor structures, and more particularly tostructures and methods of forming SiGe and SiGeC buried layers forCMOS/BiCMOS and passive devices over Si or SOI substrates. Inembodiments, the structures are formed on a silicon substrate whichincludes discontinuous buried layers having alternating regions ofsilicon and SiGe or SiGeC. In further embodiments, isolation trenchesextend through the buried layer at the Si and SiGe or SiGeC interfacesto reduce defects between the alternating regions. This SiGe(C) film,for example, can be used to form the channel/well so the dopantdiffusion can be suppressed.

More specifically, multiple structures and methods are developed inaccordance with the invention by burying SiGe and/or SiGeC films, dopedand undoped, in conventional Si and SOI wafers to improve devicecharacteristics, e.g., high voltage CMOS, PIN diode, high performanceCMOS, circuit body effect, substrate current reduction and noisecross-talk, etc. The SiGe and/or SiGeC profiling has the capabilities toallow for bandgap modulation and formation of built in electric fields.

FIGS. 1-6 show structures according to a first embodiment of theinvention. In this implementation, the SiGe or SiGeC film is formedabove a BOX. As in each of the embodiments, the SiGe or SiGeC regionsmay be non-doped, or n-typed doped or p-typed doped, depending on theparticular application.

FIG. 1 shows a starting structure comprising an optional BOX layer 12formed on a silicon wafer 10. In embodiments, the BOX layer 12 is anoxide layer in a Si substrate. The oxide buried in the silicon wafer canbe a depth ranging from less than 100 nm to several micrometers from thewafer surface, depending on the application. In embodiments, thethickness of the BOX layer 12 is in the range from about 40 nm to about100 nm. A silicon (Si) film 14 is formed on the BOX layer 12.

In FIG. 2, an optional buffer layer 16 is formed on the Si film 14. Theoptional buffer layer 16 may be Si which is grown on the Si film 14. Theoptional buffer layer 16 forms a smooth, planar upper surface forfurther processing steps, thereby preventing any defects. As furthershown in FIG. 2, a SiGe or SiGeC film 18 is formed on the buffer layer16. In embodiments, the SiGe or SiGeC film 18 can be anywhere upwards of4000 Å and in further embodiments can be about 5000 .ANG. The SiGe orSiGeC layer may be formed by conventional deposition methods such as,for example, low temperature epitaxy chemical vapor deposition (LTECVD).A Silicon (Si) epi layer 20 is formed on the SiGe or SiGeC film 18. TheGe and C composition can be in the range from any upwards to 40% and 5%,respectively.

FIG. 3 represents a selective etching process to form troughs in thestructure of FIG. 2. In this representative processing, a reactive ionetching (RIE) is undertaken to form troughs 22 to the underlying bufferlayer 16. This RIE process is provided so that in the final structureonly selective devices are formed on the SiGe or SiGeC film 18.

In FIG. 4, a second Si epi layer 24 is formed over the entire structure.This process includes filling the troughs 22 with the Si epi layer 24.

In FIG. 5, a chemical mechanical polishing (CMP) is performed toplanarize the structure. Also, in a conventional manner, isolationstructures 26 are formed between the interfaces of the Si epi layer 24and SiGe or SiGeC film 18. The isolation structures may include, forexample, shallow trench isolation structures (STI), deep trenchisolation structures (DT) and/or trench isolation structures (TI), anyof which will eliminate the defects which may exist between theinterfaces of the Si layer and SiGe or SiGeC film. In embodiments, theSTI structures may be about 4000 Å in depth, the TI structure may beabout 2.5 microns and the DT structures may be about six microns indepth, any of which depend on the specific heights applications.

FIGS. 6-13 show structures according to a second embodiment of theinvention. In this implementation, the SiGe or SiGeC film is formedabove the BOX.

FIG. 6 shows a starting structure comprising an optional BOX layer 12formed on a silicon wafer 10. In embodiments, the BOX layer 12 issimilar to that discussed with reference to FIG. 1. A silicon film 14 isformed on the BOX layer 12.

In FIG. 7, an Si epi layer 28 is formed on the Si film 14. The Si epilayer 28 is formed to make a smooth upper surface for further processingsteps, thereby preventing any defects in the final device.

As further shown, in FIG. 8 a silicon dioxide or silicon nitride layer30 is formed over the Si epi layer 28. The silicon dioxide or siliconnitride layer 30 is a hardmask, which is used for subsequent formationof troughs 22, The trough depth is preferably to the Si film 14. By wayof one illustrative example, a conventional photolithographic process isthen performed to form the trough etch openings 22, the hardmask is thenremoved from the opening as shown in the structure of FIG. 8. Thenphotoresist is stripped and troughs etch is conducted using remainingsilicon dioxide or silicon nitride layer as the hardmask. The etch depthmay be anywhere upwards of 4000 Å and in further embodiments can beabout 5000 Å.

As shown in FIG. 9, an SiGe or SiGeC film 18 is grown in the troughs 22.A silicon film 32 is formed on the SiGe or SiGeC film 18 and theremaining structure. In embodiments, the silicon film 32 may be formedby chemical vapor deposition (CVD) processes well known to those ofskill in the art. As one of skill in the art should further understand,the silicon film 32 will form a polysilicon 32 a over the silicondioxide or silicon nitride 30 since there is no seed to nucleate to formsingle crystal silicon. That is, the silicon will grow randomly formingthe polysilicon 32 a. In contrast, the silicon film 32 will remain in asingle crystal form over the SiGe or SiGeC film 18.

As represented in FIG. 10, a chemical mechanical polishing (CMP) processis performed on the structure formed in FIG. 9. The CMP will remove thedamaged areas of the polysilicon 32 a and silicon film 32.

In FIG. 11, an oxidation process is performed on the structure. In thismanner, an oxide layer (dielectric layer) 34 is formed over thestructure. The oxidation process may be performed by any conventionaloxidation growing process.

In FIG. 12, the oxide layer (dielectric layer) 34 is wet stripped usingany well known stripping process. For example, the oxide layer(dielectric layer) 34 may be stripped using wet chemical processing.

In FIG. 13, after the wet strip process, in a conventional manner,isolation structures 26 are formed between the interfaces of the Siliconlayer 28 and SiGe or SiGeC film 18. As discussed above, the isolationstructures may include, for example, shallow trench isolation structures(STI), deep trench isolation structures (DT) and/or trench isolationstructures (TI), any of which will be eliminate the defects which mayexist between the interfaces of the Si layer 28 and SiGe or SiGeC film18. In embodiments, the STI structures may be about 4000 Å, the TIstructure may be about 2.5 microns and the DT structure may be about sixmicrons, as discussed above. As further shown in FIG. 13, an Si epilayer 38 may be formed over the structure. In embodiments, the Si epilayer 38 may be anywhere upwards of about 200 to 300 Å, and morepreferably about 50 to 200 nm.

FIGS. 14-22 show structures according a third embodiment of theinvention. In this implementation, the SiGe or SiGeC film is formedbelow the BOX. FIG. 14 shows a starting structure comprising a siliconwafer 10.

In FIG. 15, a SiGe or SiGeC film 18 is formed on the silicon wafer 10.In embodiments, the SiGe or SiGeC film 18 is grown on the silicon wafer10. A silicon layer 40 is grown on the SiGe or SiGeC film 18. Thesilicon layer 40 may be about 500 Å; although the silicon layer 40 maycomprise different thicknesses as contemplated by the invention.

In FIG. 16, an oxide layer (dielectric layer) 42 is formed via an oxygenion implantation process. The oxidation process may be performed by anyconventional oxidation implantation process. For example, a conventionalSIMOX activation process is contemplated by the invention. In thismanner, oxygen ions are implanted into the Si layer to form a buriedoxide layer. The ion implantation energy is in a range of about 100 Kevto 250 kev. The oxygen dose is in a range of about 5E16 to 1E18 cm⁻².The operating temperature is in a range of about 500 to 600° C.

As shown in FIG. 17, a silicon dioxide or silicon nitride layer 44 isformed over the Si epi layer 40, which is worked as a hardmask. In aconventional manner, the hardmask is used for subsequent formation oftroughs 22, preferably to the underlying substrate. By way of oneillustrative example, a conventional photolithographic process is thenperformed over the hardmask layer to form the troughs 22. The dielectricis removed at the trough region to form the structure of FIG. 17. Theetch depth may be anywhere upwards of 4000 Å and in further embodimentscan be about 5000 Å.

In FIG. 18, a non-doped silicon film 50 is grown over the structure ofFIG. 17, including within the troughs 22. In embodiments, the siliconfilm 50 may be formed by chemical vapor deposition (CVD) processes wellknown to those of skill in the art. As one of skill in the art shouldunderstand, the silicon film 50 will form a polysilicon 50 a over thesilicon dioxide or silicon nitride 44 since there is no seed tonucleate. That is, the silicon will grow randomly forming thepolysilicon 50 a. In contrast, the silicon film 50 will remain in asingle crystal form over the SiGe or SiGeC film 18.

As represented in FIG. 19, a chemical mechanical polishing (CMP) processis performed on the structure. The CMP will remove the damaged areas ofthe polysilicon 50 a and silicon film 50 down to the level of layer 44.

In FIG. 20, a reoxidation process forms an oxide layer (dielectriclayer) 52 on the structure. The reoxidation process is a conventionalprocess, well known to those of ordinary skill in the art such asdiscussed above. The oxide layer 52 is wet cleaned using conventionalprocesses such as, for example, a DHF process using dilute HF orbuffered HF (BHF). In embodiments, the wet cleaning process removescontaminants from the wafer surface in the liquid-phase. As is known tothose of skill in the art, wet cleaning chemistries are selected to formsoluble compounds of surface contaminants; often enhanced by megasonicagitation and followed by deionized water rinse and dry cycle, forexample.

FIG. 21 shows a structure with smooth starting surface after wet cleanprocess. In FIG. 22, in a conventional manner, isolation structures 26(as discussed above) are formed between the interfaces of the Si andSiGe or SiGeC film. As discussed above, the isolation structures mayinclude, for example, shallow trench isolation structures (STI), deeptrench isolation structures (DT) and/or trench isolation structures(TI), any of which will be eliminate the defects which may exist betweenthe interfaces of the Si layer and SiGe or SiGeC film.

FIG. 23 shows a schematic representation of a band alignment inaccordance with an implementation of the invention. As schematicallyshown, due to the fact of the band gap barrier, electrons, for example,in a FET, find it more difficult to flow to the substrate due to thebarrier provided by the SiGe (or SiGeC) doped areas of the structure. Inthis manner, the SiGe (or SiGeC) doped areas will direct the electronsinto the source of the device which is normally grounded, blocking theelectrons from flowing into the substrate. This will substantiallydecrease device leakage to the substrate or prevent the body currentgenerated from other parts flow to this device.

FIG. 24 shows a graph depicting out diffusion of Boron in a structureimplementing aspects of the invention. In this graph, the diffusion ofBoron is significantly decreased over SiGe (or SiGeC) doped areas of thestructure. More specifically, out diffusion of Boron significantlydecreases at SiGe (and SiGeC) doped areas. In this manner, in scaleddevices implementing the structures of the invention, where the sourceand drain are very close together (e.g., 0.1 micron or less), the boronwill not out diffuse and cause shorting of the device.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

By way of illustration, referring to FIGS. 25 and 26, final devices canbe formed on the structures of the invention. For example, as shown inFIG. 25, a CMOS device may be formed on the above BOX structuresdescribed with reference to FIGS. 1-13. In FIG. 26, an HB NPN HBTstructure 100 may be formed on the structures of the invention, as wellas passive devices such as varactors and PIN diodes 200.

While the invention has been described in terms of exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modifications and in the spirit and scope of theappended claims.

1. A method of forming an integrated structure, comprising: etchingtroughs in a continuous layer of Si material formed over a substrate andhardmask that was formed over the Si material; depositing in the troughsSiGe or SiGeC material to form alternating Si regions with SiGe or SiGeCregions; forming a Si film over the Si material and the SiGe or SiGeCmaterial, wherein the Si film forms polysilicon over the hardmask;removing damaged portions of the Si layer and the polysilicon layer;forming isolation structures at interfaces between the SiGe or SiGeCregions and the alternating Si regions; forming a buried oxide layerover the substrate; and forming an epitaxial layer over isolationstructures, SiGe or SiGeC regions and Si material after an oxidationprocess.
 2. The method of claim 1, wherein the buried oxide layer isformed one of under the SiGe or SiGeC regions and over the SiGe or SiGeCregions.
 3. The method of claim 2, wherein the SiGe or SiGeC regions arenon-doped, or n-typed doped or p-typed doped.
 4. The method of claim 3,further comprising forming a first type device over the Si regions and asecond type device over the SiGe or SiGeC regions.
 5. The method ofclaim 1, wherein the isolation structures extend into the buried oxidelayer.